1. Field of the Invention
The present invention relates to an apparatus and a method for calibrating phase difference, and more particularly, to an apparatus and a method for calibrating IQ mismatch.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 illustrates a diagram of a conventional direct down-conversion receiver 10. The direct down-conversion receiver 10 includes an antenna 11, a low noise amplifier (LNA) 12, mixers 14, 24, low pass filters (LPF) 16, 26, analog to digital converters (ADC) 18, 28, and a digital signal processor (DSP) 19. The antenna 11 receives a radio communication signal, and the LNA 12 is utilized for amplifying said radio communication signal received by the antenna 11. The mixer 14 mixes the radio communication signal with a first carrier wave (COSωct as shown in FIG. 1) to generate an analog signal Sa1, whereas the other mixer 24 mixes the radio communication signal with a second carrier wave (SIN(ωct+δ) as shown in FIG. 1) to generate an analog signal Sa2. The LPFs 16, 26 filter out high frequency components of the analog signals Sa1, Sa2 respectively. Furthermore, the ADCs 18, 28 respectively convert the analog signals Sa1, Sa2 into corresponding digital signals Sd1, Sd2. Lastly, the DSP 19 is utilized for succeeding signal processing on the digital signals Sd1, Sd2.
As known by those skilled in the art, a 90-degree (i.e. orthogonal) phase difference is required between the first carrier wave and the second carrier wave so that the mixed analog signals Sa1, Sa2 are orthogonal to each other, where the analog signal Sa1 is an in-phase signal and the analog signal Sa2 a quadrature-phase signal. However, in a practical circuit, factors such as temperature, manufacturing process, supply voltage drift, etc., cause an ideal orthogonal phase difference, which indicates 90-degree, to have a phase deviation δ between the first carrier wave and the second carrier wave. This phenomenon is known as IQ mismatch. As shown in FIG. 1, a phase deviation δ exists between the first carrier wave COSωct and the second carrier wave SIN (ωct+δ). The IQ mismatch affects signal demodulation so that a bit error rate of a communication system is raised. Therefore, the above-mentioned phase deviation δ has to be calibrated for further correcting the analog signals Sa1, Sa2 so as to increase the bit rate of the communication system.
There are two methods of calibrating the IQ mismatch of the conventional direct down-conversion receiver. In a first conventional method, after an in-phase analog signal and a quadrature-phase analog signal are converted into a corresponding in-phase digital signal and a corresponding quadrature-phase digital signal by the analog-to-digital converters 18 and 28 respectively, a phase deviation δ between said two digital signals are measured in the digital domain. Afterwards, an adjustment signal is outputted according to the measured phase difference, and a phase compensation is performed on both the in-phase analog signal and the quadrature-phase analog signal in the analog domain for compensating IQ mismatch between said analog signals. In a second conventional method, a phase deviation δ between the in-phase digital signal and the quadrature-phase digital signal are also measured in the digital domain. A difference between the first and the second conventional methods is, after measuring said phase deviation in the second conventional methods, a phase compensation is immediately performed on both the in-phase digital signal and the quadrature-phase digital signal in the digital domain. In both the above-mentioned conventional methods, the phase deviation between the digital signals Sd1, Sd2 is measured in the digital domain by performing a Discrete Fourier Transform (DFT) on the digital signals Sd1, Sd2 with a digital circuit of the DSP 19 for determining the phase deviation δ. Afterwards, a conventional Gram-Schmidt orthogonalization method is utilized to perform a phase compensation in the analog domain. A least-mean-square (LMS) algorithm implemented with a digital circuit may otherwise be utilized to perform a phase-compensation in the digital domain. For a detailed explanation of the abovementioned phase compensations, please refer to “Adaptive IQ Mismatch Cancellation for Quadrature IF Receiver,” Isis Mikhael, Wasfy B. Mikhael, David Chestr, and Brent Myers, IEEE Midwest Symposium on Circuits and Systems, August 2002. However, utilizing the DFT to calculate the phase deviation δ not only requires a complex logic circuit to perform inextricable logic calculations, but also raises power consumption. Furthermore, a digital circuit requires external calibrating signals to perform the LMS algorithm for the phase compensation, and which complicates related circuit design and raises related power consumption.